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Old 2020-07-21, 12:47   #8
rogue's Avatar
Apr 2003
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6,277 Posts

Originally Posted by gd_barnes View Post
Where is the workaround that you have stated? I do not see it. I must be dense. lol We do not want to have to input multiple sequences if we only want to sieve one of them. I don't understand.

Please provide a specific example of how we should sieve (13*51^n-1)/2 by sieving 13*51^n-1. Please state the program to use and the command to use. Assume I'm a newbie because I am when it comes to srsieve2.

No we've never asked for srsieve (or srsieve2) to sieve (k*b^n+c)/d. I was just showing an example of what we could sieve with srsieve if the error check for divisibility by 2 is removed. We are just asking for the error check for all candidates divisible by 2 to be removed and then be included in the regular sieving looping process of all other prime factors.

I have used srsieve2 a few times. I find it to be no faster than sr2sieve and find it somewhat more cumbersome to use. I don't care for the fact that the P-rate that it uses is the number of prime factors that it has searched in the last minute whereas the P-rate in srsieve is the P-range that it has searched in the last minute. IMHO the latter is much preferable. I still use srsieve/sr1sieve/sr2sieve for that reason.

Why are you suggesting that we use srsieve2 when you do not know if it will work for what we are asking?

I just tried srsieve2 with the following command:
srsieve2 -p 3 -P 1e6 -n 25e3 -N 100e3 -s "13*51^n-1"

It errored out with:
All terms for sequence 13*51^n-1 are divisible by 2
Fatal Error: All sequences have been removed

So my question is: Why does it error out when I have not included p=2 in the the sieve range ? It should not do that. For the same reason that srsieve should not do that. Sieving 13*51^n-1 by not including the factor of 2 is our way of sieving (13*51^n-1)/2. Does that make sense?
For srsieve2 I see what you mean. The check for "divisibility by 2" is done before sieving is started.

I will change srsieve2 to sieve (k*b^n+c)/d for any d. That would allow srsieve2 to benefit other users as it will be able to sieve a few repunit sequences.

Regarding "p per second", the value it shows is rather useless. The factor rate and estimated completion time are far more important.

For srsieve I will change the current error condition to a warning (it is a warning in srsieve2), but cannot guarantee that starting with -p3 will not remove terms divisible by 2.
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